1. Field of the Invention
The present invention relates to a timing recovery circuit and a timing recovery method.
2. Description of the Related Art
In some wireless or high-speed data communication systems, signals arriving at a receiver by different paths show different time delays. This results in inter-symbol interference (ISI), a degradation in reception. Amplitude and phase of a received signal may be severely distorted, and may cause a bit error in the receiver.
For example, one high-speed data communication system where this is of concern is in systems employing Digital Subscriber Line (DSL) technology. DSL, which provides high-speed communications using telephone lines, requires the use of a wide band of frequencies to send more information than conventional voice calls require. Wideband modulation schemes need to take into account the broadband characteristics of the medium used to communicate. Twisted pair copper wire subscriber loops used to provide access to the local exchange in telephone circuits exhibit frequency dependent attenuation, with higher frequencies suffering more attenuation than the lower ones. Further, these subscriber loops also exhibit a non-linear phase response with frequency, with the lower frequencies exhibiting more non-linearity. This kind of channel characteristic results in the dispersion of an impulse, sent by the transmitter, at the receiver, thus corrupting the reception. This dispersion is referred to as ISI, and results in data loss and hence, loss of communication reliability.
A channel's limited bandwidth has a dispersive effect on the transmitted pulse. High-frequency loss in the channel tends to reduce the slope of the pulse edges resulting in finite rise times, known as ‘precursor distortion’. At low frequencies, the nonlinear phase characteristics tend to produce a substantially long decay tail, or ‘postcursor distortion’, resulting in a smeared pulse shape. The “available” bandwidth and the phase characteristics of a channel are thus a function of the medium used. Distortion suffered by a given pulse results in interference to its neighbors in time. A given pulse is distorted by the presence of tails from past pulses and precursors of future pulses. This effect is known as the aforementioned ISI. Thus, at the receiver, detection of symbols is further complicated by the presence of pulse distortion in addition to noise. For a given channel, since the channel attenuation and noise characteristics can be determined a priori, ISI may be eliminated by predicting the channel's future and past influence on any received symbol. The process of eliminating ISI from the received data is referred to as equalization.
An equalizer of the receiver typically compensates the distortion due to ISI. The equalizer compensates amplitude and delay of the received signal in an effort to enhance the quality of the communication channel without increasing the power of transmitting signal, or without increasing channel bandwidth. In general, since channel properties are not known with exactness and vary over time, the receiver employs an adaptive Decision Feedback Equalizer (DFE) that changes a tap coefficient value of the equalizer, depending on the channel properties.
In a receiver of a general data communication system, an ADC (Analog-to-Digital Converter) converts a received continuous-time signal to a discrete-time signal. A clock of the receiver should be synchronized with a clock of a transmitter of the general data communication system. The most common circuit that extracts timing information (for example, frequency and/or phase information of the clock of the transmitter) is a Phase-Locked Loop (PLL). The PLL typically generates the clock of the receiver. The receiver traces the frequency and phase of the clock of the transmitter to compensate the clock of the receiver. The received signal is sampled using the clock of the receiver and is converted to digital data. A timing recovery circuit synchronizes the clock of the receiver with the clock of the transmitter. In a mobile communication system, a pilot signal is utilized to synchronize the clock of the receiver with the clock of the transmitter.
In digital data communication systems, the receiver extracts timing information from received data. One of the conventional methods of extracting timing information from the received data is disclosed in an article by Mueller et al., entitled, “Timing recovery in digital synchronous data receiver”, IEEE Transactions on Communications, pp 516-531, Vol. 24, May 1976, which is incorporated by reference herein in its entirety. In this paper the authors propose a timing recovery algorithm referred to as an “M&M algorithm”. The paper is accepted in the art as the basis for timing recovery algorithms.
The M&M algorithm may be embodied as hardware and has a stable loop property. However, when using a channel that severely distorts signals being transmitted through the channel, the M&M algorithm must utilize a shaping filter so that input signals have symmetric properties. In addition, timing errors increase for the severely distorted channel, according to the M&M algorithm.
Another method of extracting the timing information is through a cross-correlating precursor and timing error process. This process requires an exact control of a loop filter, however, since either the precursor or the timing error is distorted based on the channel status. Thus, extracting timing information by the cross-correlating precursor and timing error process requires substantial hardware.
FIG. 1 is a schematic view illustrating a prior art timing recovery method. Referring to FIG. 1, to detect timing errors, the prior art timing recovery method extracts a precursor and a postcursor using a difference between a coefficient value of a Feed-Forward Filter (FFF) of the DFE and a coefficient value of a FBF (Feed-Back Filter) of the DFE.
A precursor represents a signal component corresponding to a front part of an impulse response of a channel (i.e., communication channel) with respect to a signal peak value of the impulse response of the channel. A postcursor represents a signal component corresponding to a rear part of the impulse response of the channel with respect to the signal peak value. ISI may occur due to an overlap between the precursors and the postcursors of at least two neighboring symbols.
The purpose of the timing recovery is to find out the zero-crossing, or generally to find out a transition point of a signal. The zero-crossing represents a zero-crossing of a detected timing error. Thus, the zero-crossing may vary depending on the factor(s) a designer may use so as to detect the timing error. Namely, a desired timing phase may vary depending upon internal filters of the timing recovery circuit and environments, and the algorithm of extracting the timing errors may vary depending upon the applications, for example.
Used for timing recovery, the precursor may be considered as a zero-crossing indicator inserted at a precursor position of a received signal. Such a zero-crossing assists a timing recovery circuit in determining phase relationships between signals, by giving the timing recovery circuit an accurately determinable signal transition point for use as a reference based on future data symbols. A postcursor may be considered as another determinable signal transition point for use as a reference that is based on past data symbols.
The converging properties of the DFE should be such that the DFE may be able to correctly output an estimate of the ISI present in incoming signal samples based on the sequence of past decisions. This ISI represents interference from past data symbols, and is commonly termed postcursor ISI. After convergence of the DFE, the DFE can accurately estimate the postcursor ISI.
An operator 102 subtracts a coefficient value of the FBF from a coefficient value of the FFF. The operation block 104 of FIG. 1, which is known in the art and therefore not described in detail, processes the result of the operator 102 to change phase of the clock. A timing error may thus be extracted from a difference between the coefficient value of the FBF and the coefficient value of the FFF, since the FFF compensates the precursor and the FBF compensates the postcursor, and since coefficient values may vary in proportion to the phase variation of a changed clock.
According to the method as shown in FIG. 1, an initial phase stability is enhanced and a pulse shaping filter is not required, since a timing restoring loop converges to a stable value depending on the converging properties of the DFE. However, the converging properties of the coefficients of the DFE may be greatly affected by the initial phase of a PLL, in the case a PLL is used to extract timing information. The coefficients of the DFE have unstable converging properties and do not converge to a stable value when the initial phase has the property in which the difference between the coefficient value of the FBF and the coefficient value of the FFF is maintained, in addition to a small timing error being maintained with respect to the initial coefficient value of the DFE.
FIG. 2 is a graph illustrating converging properties of the coefficients of a DFE according to a worst initial phase, based on the prior art method of FIG. 1. A worst initial phase is when the difference between the coefficient value of the FBF and the coefficient value of the FFF is relatively small, but the difference between the coefficient value of the FBF (FFF) and the initial coefficient value of the FBF (FFF) is relatively large. Referring to FIG. 2, the upper graph (A) illustrates a converging property of the coefficient value of the FFF according to a worst initial phase, and the lower graph (B) illustrates a converging property of the coefficient value of the FBF according to the worst initial phase.
In graph (A), the y-axis represents a symbol time, the x-axis represents the coefficient value of the FFF, the dotted line represents the FFF coefficient value having ideal converging property, and the solid line represents a FFF coefficient that fails to converge to an approximately constant value. In other words, the FFF coefficient of the solid line does not approach a stable value. Similarly in graph (B), the y-axis represents a symbol time. The x-axis represents the coefficient value of the FBF, the dotted line represents the FBF coefficient having ideal converging property and the solid line represents the FBF coefficient that fails to converge. The FBF coefficient of the solid line thus does not approach a stable value. Accordingly, the converging property of the DFE may be inadequate, depending on the initial phase of timing recovery; thus the prior art method of FIG. 1 requires a control block for controlling the initial phase, as described by the M&M algorithm.
FIG. 3 is a block diagram illustrating a prior art error detector for timing recovery. This error detector is disclosed in PCT Laid-open Publication No. WO 01/52469, published and entitled “Baud-rate timing recovery”. Referring to FIG. 3, an error detector receives coefficient values of the FFF and FBF from a DFE of a receiver, and calculates a timing error that is output to a loop filter.
Referring to FIG. 3, a first postcursor b(1) is input to an amplifier 301 where it is scaled (α1×(b1). Precursors w(N−1) and w(N−2) are inputted to a divider 302 to be divided to a result w(N−2)/w(N−1), which is input to an amplifier 303, and scaled as β1×w(N−2)/w(N−1). An adder 304 sums outputs of amplifiers 301 and 303 to output a timing function z1(n). The timing function z1(n) is shown in expression 1.
                                          z            ⁢            1                    ⁡                      (            n            )                          =                                                            a                ⁢                1                            ×                              (                                  b                  ⁢                  1                                )                                      +                          β1              ×                                                w                  ⁡                                      (                                          N                      -                      2                                        )                                                                    w                  ⁡                                      (                                          N                      -                      1                                        )                                                                                =                                                    α1                ⁡                                  (                                      h                    ⁢                    1                                    )                                            -                              β1                ⁢                                                                  ⁢                                  h                  ⁡                                      (                                          -                      1                                        )                                                                                      h              ⁡                              (                0                )                                                                        (        1        )            
According to FIG. 3, since the scale factors α1, β1 should be estimated based on asymmetric property of input signals, impulse response of specific channels should be known. Therefore, large timing errors may occur for unknown channels or for properties of a channel that vary over time.